This invention relates to an electrochemical sensor, and particularly to an electrochemical sensor based on a conbination of a membrane having an immobilized enzyme capable of decomposing a substrate in a sample, thereby forming hydrogen peroxide (H.sub.2 O.sub.2) and an electrode capable of detecting hydrogen peroxide.
It is known that the conventional electrochemical sensor can quantitatively determine trace components such as glucose, urea, cholesterol, etc. contained in a body fluid, living body tissue, food, etc. with a good selectivity.
The electrochemical sensor has such a distinguished characteristic that no special reagent is required, and its application field has been expanded with the recent development of living body catalyst-immobilizing technique.
According to the structure of the electrochemical sensor, a resin layer having an immobilized living body catalyst consisting of an enzyme is provided on the outer peripheral surface of the sensor, and a working electrode for determining a concentration of the reaction product formed by reaction of a substrate in a sample with the living body catalyst by detecting an electrical change (e.g. changes in potential and current) is provided inside the sensor.
An example of the conventional and ordinary immobilized enzyme electrode is disclosed, for example, in Japanese Patent Application Kokai (Laid-open) No. 55-98347, where a working electrode is extended to the center of an enzyme electrode so that the tip end of the working electrode can come in substantial contact with the surface of the immobilized enzyme membrane.
According to a method for determining a substrate in a sample by an immobilized enzyme electrode, electrode-active substances such as hydrogen peroxide, ammonium ions, carbon dioxide gas, etc., formed by reaction of the enzyme with the substrate when the enzyme electrode is brought in contact with a sample solution, are amperometrically or potentiometrically detected to determine unknown quantities of substances contained in the sample solution. That is, the current or electromotive force generated on the electrodes is proportional to a quantity of a substance to be determined in a sample, and thus the substance in the example can be quantitatively determined from a working curve prepared in advance.
An application example of said enzyme electrode is a glucose electrode practically used for amperometrically determining glucose in blood or urine by decomposing the glucose to hydrogen peroxide by immobilized glucose oxidase and measuring the concentration of hydrogen peroxide with an internal electrode having a novel metal as a working electrode, to which about 0.6 V is applied in advance. The glucose electrode is incorporated into a unifunctional apparatus mainly based on a batch-type measurement, and is extensively used as an important information source for the diabetes mellitus and other diseases.
To meet the requirements for rapidity in the quantitative determination of a glucose concentration, a glucose sensor has been just applied to a flow-through type analyzer [e.g. "Analytical Chemistry, Vol. 51 No. 1 (1979) pages 96-100"]. The flow-through type determination comprises introducing a sample of blood or urine from a patient into a carrier liquid, and bringing the carrier liquid containing the sample in contact with a glucose electrode provided in the flow passage to quantitatively determine the concentration of glucose in the sample.
The present inventors have found that an application of the immobilized enzyme electrode as disclosed in said Japanese Patent Application Kokai (Laid-open) No. 55-98347 to a flow-through type analyzer as such has various practical problems. One of the problems is a phenomenon that, when a glucose electrode is left in contact with a liquid for a long time, an abnormally high measured value is obtained in the successive measurement of an actual sample. If an operator carries out analytical operation without any knowledge of the phenomenon found by the present inventors, elevation of the base line is induced to lower the accuracy of measurement.